For example, shaft blanks such as for a camshaft embedded in an engine are mainly formed by means for forging. In such a shaft blank, cutting and polishing processes are executed for the outer peripheral surfaces of cam portions and journal portions.
As a process preceding the aforementioned processes for the shaft blank, it is required to bore center holes as references for machining in the both end surfaces of the shaft blank. A variety of methods have been proposed as the methods of machining center holes.
In a method of Japan Laid-open Patent Application Publication No. JP-A-H09-174382, for instance, a true center position of a shaft blank is predicted based on the data obtained by measuring the shape of the shaft blank, and a center hole is bored in the true center position obtained by the prediction. Subsequently, machining is executed for the outer peripheral portion and etc. of the shaft blank using the center hole.
On the other hand, in a method of PCT International Publication No. WO2009/016988A1, the three-dimensional shape data of a shaft blank (crankshaft blank) is obtained and a center hole position of the shaft blank is temporarily determined based on the three-dimensional shape data. Next, machining of the shaft blank is simulated based on the temporarily determined center hole position. The simulated post-machining shape is specified for the shaft blank. Further, it is determined whether or not the simulated post-machining rotational balance of the shaft blank falls within an allowable range. When it is determined that the rotational balance falls within the allowable range, the temporarily determined center hole position is set as a center hole position for actual machining.